1. Field of Invention
The present invention relates to a constant on-time switching regulator, and a control method and an on-time calculation circuit therefor.
2. Description of Related Art
Referring to FIGS. 1A and 1B, a prior art constant on-time (also referred to as “Ton”) switching regulator includes an upper gate power switch UG and a lower gate power switch LG, which are respectively controlled by gate signals Vug and Vlg from driver gates 21 and 22 to convert an input voltage Vin to an output voltage Vout. The constant Ton switching regulator operates as follows: When a feedback voltage Vfb representing the output voltage Vout is lower than a predetermined voltage Vref, a comparator Com generates a comparison signal Vcom with low level, wherein one edge of the comparison signal Vcom determines a starting point of the on-time of the upper gate power switch UG (in a real case, the starting point may be slightly later than the triggering edge of the comparison signal Vcom due to circuit delay, which is omitted in the figure). A control circuit 11 controls the operations of the upper gate power switch UG and the lower gate power switch LG according to the triggering edge of the comparison signal Vcom. A constant Ton circuit 13 generates a signal determining a constant on-time. A one-shot pulse generator 12 turns on, according to a signal from the control circuit 11 and the signal from the constant Ton circuit 13, the upper gate power switch UG for the constant on-time determined by the constant Ton circuit 13. When the on-time is over, the control circuit 11 turns OFF the upper gate power switch UG and turns ON the lower gate power switch LG, until the next time when the feedback voltage Vfb is again lower than the predetermined voltage Vref, and the control circuit 11 turns OFF the lower gate power switch LG and turns ON the upper gate power switch UG ON again. The control circuit 11 operates periodically as thus. In this prior art, because the starting point of the on-time of the upper gate power switch UG depends on the timing when the feedback voltage Vfb is lower than the predetermined voltage Vref, but the on-time is fixed without modulation, the switching regulator does not operate in a fixed-frequency.
In the above circuit, the comparator Com can be replaced by an error amplifier, that is, the comparison signal Vcom can be a digital signal or an analog signal, and the control circuit 11 can be different structures correspondingly.
Referring to FIG. 2A which shows another prior art constant Ton switching regulator, in order to operate the constant Ton switching regulator by fixed-frequency, this prior art proposes to provide a Ton calculation circuit 14 which calculates a proper Ton according to the input voltage Vin and the output voltage Vout, such that the circuit can operate in a fixed-frequency. Referring to FIG. 3, in a continuous conduction mode, when the upper gate voltage Vug is at high level (ON), the lower gate voltage Vlg is at low level (OFF); when the upper gate voltage Vug is at low level (OFF), the lower gate voltage Vlg is at high level (ON). A phase voltage Vph at a node between the upper gate power switch UG and the lower gate power switch LG follows the waveform of the upper gate voltage Vug. In an ideal case, when the upper gate power switch UG is ON, the phase voltage Vph is equal to the input voltage Vin; when the lower gate power switch LG is ON, the phase voltage Vph is equal to 0. In other words, in the ideal case, the ratio of the input voltage to the output voltage is equal to the duty ratio D of the upper gate voltage Vug, that is, the ratio of Ton to period T. Thus, if the period T of the constant Ton switching regulator can be controlled by the following way:
      T    =          Ton      ×              Vin        Vout              ,then the circuit can operate in a fixed-frequency.
According to the above, the prior art proposes the Ton calculation circuit 14 shown in FIG. 2B, wherein a current source Cs1 generating a current K-times of the input voltage Vin (i.e., the current source Cs1 generates a current of K*Vin) charges a capacitor C1 having a capacitance of C to generate a voltage Vc across the capacitor. A comparator Com1 compares the voltage Vc and the output voltage Vout to generate a square wave signal. According to the equation t=CV/I (time=capacitance*voltage/current), the Ton calculation circuit 14 generates the desired Ton for the upper gate power switch UG when the comparator Com1 outputs a high level signal, as the following:
  Ton  =            C      K        ×                  Vout        Vin            .      In other words, in the ideal case, Ton=(C/K)×D, and Ton can be set to a proper value by adjusting K and C such that the constant Ton switching regulator operates in a fixed-frequency.
However, in a real case as shown in FIG. 4, the high level and low level of the phase voltage Vph are not Vin and 0. Due to the turn-ON resistances of the upper gate power switch UG and the lower gate power switch LG, the high level and low level of the phase voltage Vph are actually Vin-IL*Rug and −IL*Rlg, where IL is the load current and Rug and Rlg are the turn-ON resistances of the upper gate and lower gate power switches, respectively. Thus, under the prior art structure in FIGS. 2A-2B, the output voltage Vout is not equal to Vin*D actually, that is,
  Ton  =                    C        K            ×              Vout        Vin              ≠                  C        K            ×              D        .            Therefore, the Ton calculation circuit 14 designed under the ideal case assumption cannot achieve fixed-frequency operation.